Expert knowledge is highly idiosyncratic:

• Build stair steps 7 inches high and 10 inches wide.
• Two times height plus width should equal 25 inches.
• Width times height should equal 72 inches.[Parker, T., Rules of Thumb, Boston, MA: Houghton Mifflin Publishers, 1983.]

• Different rules may be generated for the same phenomena.
• The rules may have no fundamental validity and may give bad answers outside a limited domain of applicability.
• The rules generally work within the limits of applicability, but the expert often doesn't know what the limits are.

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